In the cultivation of fruit trees of various kinds, it is essential to carry out preliminary work of picking off some of the flowers or fruit which are not required to fructify. This work of picking off the unrequired bodies of flowers or fruit is known as "thinning" of the flowers or fruit. Meanwhile, this thinning of the flowers or fruit usually requires a large amount of labor by workers. The thinning work must be done usually in the busy seasons of agriculture, and hence it is frequently difficult to carry out the thinning work at an appropriate time, so that the quality of the fruits cultivated and harvested is therefore sometimes lowered.
Heretofore, the methods for thinning the flower or fruit of fruit trees by chemical compounds have been extensively researched for almost all kinds of fruit trees such as apple tree, peach tree and others, and there has been proposed and developed a number of chemical thinning agents. The known chemical thinning agents may be classified according to their biological mechanisms of the action of these agents, and they may generally be classified into two groups, that is, a first group of such agents which act as a plant hormone and the second group of such agents which can exert phytotoxicity against the bodies or different organs of the flowers of fruit trees. While, such chemical thinning agents which have been extensively applied in practice for partial removal of the unrequired flowers or fruits are limited only to the formulation known under the name "Ethyclozate" for use in the thinning treatment of tangerine tree; the formulation known under the name "Sevin" for use in the thinning treatment of apple tree; and certain sulfur preparations for use in the thinning treatment of apple tree. The active components of the thinning agents known under the names "Ethyclozate" and "Sevin" belong to a class of plant hormone in nature, so that their flower-thinning effects can vary unfavorably depending on the biological conditions, of the trees treated and also on the weathering conditions, which is specifically observed with the plant hormone agents in general. On the other hand, the sulfur preparations practically employed as the chemical thinning agent are active in inhibiting the fertilization of flowers owing to its phytotoxicity to the stigma of the flower body, so that its flower-thinning activity is normally lower than those of the flower-thinning agents of plant hormone type. When the sulfur preparations are applied in an increased rate of application in an attempt to enhance its flower-thinning effects, the leaves and other portions of the fruit trees treated can often be damaged by the phytotoxicity of the sulfur preparations
For these reasons, there exists a great demand for a new flower-thinning chemical agent which is free from the above-mentioned drawbacks of the known chemical flower-thinning agents. In an attempt to meet this demand, we previously investigated a number of chemical compounds which were different in nature from the active components of the known thinning agents, and we already found that 2-pyrrolidone-5-carboxylic acid and salts thereof such as the sodium salt; and salts of an N-acylaminodicarboxylic acid; as well as esters of sucrose with a higher fatty acid and plant sterols were each active in thinning the flowers of the cultivated fruit trees. Flower-thinning agent comprising the above-mentioned compounds as the active component are disclosed in the specifications of European Patent Publication No. 0089205 A2 (published on Sept. 21, 1983) and U.S. Ser. No. 473,354.
We have further investigated in an attempt to find out other chemical compounds which are effective in the thinning of flowers and fruit and we have now found that cholesterol is active as a flower-thinning agent.